egl: put full path to libEGL_mesa.so in GLVND json

This is useful when installing to a non-standard path.

glvnd_icd.py copied & adapted from src/intel/vulkan/anv_icd.py

Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Acked-by: Matt Turner <mattst88@gmail.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3038>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3038>

src/egl/meson.build

@@ -152,8 +152,18 @@ else
   deps_for_egl += dep_glvnd
   files_egl += [g_egldispatchstubs_h, g_egldispatchstubs_c]
   files_egl += files('main/eglglvnd.c', 'main/egldispatchstubs.c')
-  install_data(
-    'main/50_mesa.json',
+
+  egl_icd = custom_target(
+    'glvnd_icd',
+    input : 'glvnd_icd.py',
+    output : '50_mesa.json',
+    command : [
+      prog_python, '@INPUT@',
+      '--lib-path', join_paths(get_option('prefix'), get_option('libdir')),
+      '--out', '@OUTPUT@',
+    ],
+    build_by_default : true,
+    install : true,
     install_dir : join_paths(get_option('datadir'), 'glvnd', 'egl_vendor.d')
   )
 endif